Refactor nunit tests off InMemory/WebApplicationFactory, remove tuple returns (#4)

## Summary
- Extract `IMicCheckDbContext` and mock DB access with Moq instead of the EF InMemory provider, per CLAUDE.md testing guidelines.
- Rewrite HTTP-pipeline tests to exercise controllers/auth handlers directly instead of `WebApplicationFactory`.
- Remove tuple return types across the API in favor of named records (`PagedResult<T>`, `ProfileUpdateResult`, `ApiKeyCreationResult`).

## Test plan
- [x] `dotnet build` (full solution)
- [x] `dotnet test tests/api/MicCheck.Api.Tests.Unit` — 187/187 pass
- [x] `dotnet test tests/api/MicCheck.Api.Tests.Integration` — 8/8 pass (live API + Postgres via Aspire)
- [x] Verified Aspire AppHost/dashboard starts and API responds

Reviewed-on: #4

src/api/MicCheck.Api/Features/FeatureStateService.cs

@@ -5,7 +5,7 @@ using Microsoft.EntityFrameworkCore;
 
 namespace MicCheck.Api.Features;
 
-public class FeatureStateService(MicCheckDbContext db, WebhookQueue webhookQueue, AuditService auditService)
+public class FeatureStateService(IMicCheckDbContext db, WebhookQueue webhookQueue, AuditService auditService)
 {
     public async Task<IReadOnlyList<FeatureState>> ListByEnvironmentAsync(int environmentId, CancellationToken ct = default)
     {